It is common practice to provide a printed circuit board, along at least one of its edges, with a series of conductive paths lying parallel to one another and perpendicular to the edge of the board. These conductive paths end in terminal connection zones or conductor pads and form an integral part of the printed circuit wiring on the board, i.e., the conductor pads are produced from the same layer of conductive material as the printed circuit interconnection wiring and are produced by the same process of fabrication. The conductor pads are, consequently, very fragile, and this causes problems in making connection with other conductors or connectors of an electrical apparatus.
Specifically, it is extremely difficult to insert directly the edge of a circuit board into a female connector having a large plurality of contacts of the lyre type, such as is commonly in use today. In the lyre type of connector, the resilient contact blades are shaped to receive one end of a board and to clamp the end of the board in a connector body of insulating material. The free ends of the contacts are generally bent inwardly and then outwardly and upwardly so as to offer, prior to insertion, a lesser width of passage than the width of the board itself. In this manner, by applying an insertion force against the board, the curved parts of the receiving contacts are displaced and apply lateral contact pressure forces upon the board. However, as the board is inserted, these lateral forces induce frictional forces upon the fragile contact pads of the board. The same deleterious effect is produced during extraction of the board. Since the resilient contact blades of the female connector are designed to exert lateral forces sufficient for establishing an electrical contact by pressure between the blades and the conductive pads of the board, the induced frictional forces are relatively large. Accordingly, the characteristics of the contact which enhance good electrical and mechanical connection, e.g., large lateral forces, are opposed to the characteristics which aid in reducing chipping and wear of the conductor pads on the board. The lateral forces thus act against the ideal principals of connection of circuit boards with mating contacts, i.e., to offer non-limiting and non-destructive extraction and insertion of the board.
It has been proposed to use an intermediate element in the shape of a connection bar fixed to the end of a printed circuit board, for example with screws, the intermediate element having terminals in the shape of tongues or pins and being permanently connected, as by soldering, with the conductor pads of the board. Connection is then made by means of the stronger terminals of this intermediate element attached to the board.
The use of an intermediate element has obvious disadvantages. For example, it constitutes a supplementary piece which adds to cost, maintenance, assembly, etc. In order to avoid the need for any intermediate element, a process has been proposed consisting in the hardening of the mechanical characteristics of the connector pads on the printed circuit board by coating the pads by a galvanic deposit process which, without harming the electrical qualities of the board, permits introduction of the board directly into a connector of the above-mentioned type with a relatively reduced rate of adverse incidences. However, in order to obtain simultaneously the characteristics of good electrical conduction and good mechanical hardness, the galvanic deposit is generally of rare metals such as nickel and/or rhodium, ie., materials that are mechanically more resistant to wear than the usual red copper of the printed circuit board, but equally more expensive.
Additionally, the coating process must be carried out with extreme precision in order to avoid improper depositing techniques which make the thus obtained contact surfaces abrasive and/or make the contacts susceptible of rapid deterioration during the process. The required precision and high quality standards of the process increase considerably the cost of production of such printed circuit boards.
It is often preferred in the prior art to deposit a less costly alloy of tin and lead instead of the rare metal deposit discussed above. The tin and lead alloy, if it covers the entire conductive wiring paths on the circuit board, has the additional advantage of effecting directly in a single operation the tinning of all of the copper parts of the board. This type of coating reduces the risks of flaking or scraping of the conductive terminal pads on the board, but it presents little advantage over bare copper itself, since it is extremely ductile and consequently not able to support large mechanical friction forces. Consequently, the utilization of the lyre type connectors is equally problematical with this type of coating, since the frictional forces developed during the insertion and extraction of the board are not compatible with the mechanical endurance of the coating.